This invention relates generally to a joining strategy for batteries, and more particularly to one-sided ultrasonic bonding of battery tabs and bus bars that are used for vehicular and related transportation applications such that high power, two-sided welding methods are avoided.
Lithium-ion and related batteries are being used in automotive and related transportation applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes such batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. The flow of electric current to and from the individual cells (i.e., a single electrochemical unit) is such that when several such cells are combined into successively larger assemblies (such as modules and packs), the current or voltage can be increased to generate the desired power output. In the present context, larger module and pack assemblies are made up of one or more cells joined in series (for increased voltage), parallel (for increased current) or both, and may include additional structure to ensure proper installation and operation of these cells. One common vehicular form of the battery pack is known as a power battery, while another is known as an energy battery.
In one form, the individual cells that make up a battery pack are configured as rectangular (i.e., prismatic) cans that define a rigid outer housing known as a cell case. These types of cells are generally assembled into the power battery pack variant. In another form, the individual cells are housed in thinner, flexible rectangular pouches that are generally assembled into the energy battery pack variant. Both cell types can be placed in a facing arrangement (much like a deck of cards) along a stacking axis formed by the aligned parallel plate-like surfaces. Positive and negative terminals situated on one edge on the exterior of the housing of each cell are laterally-spaced from one another to act as electrical contacts for connection (via bus bar, for example) to an outside load or circuit. With particular regard to the prismatic pouch, the battery cells may incorporate thin metal sheets as electrode substrates, or simply electrode sheets, to generate the flow of electric current. These electrode sheets incorporate an extension, i.e., tab, which extends outside of the cell pouch and is used to join the electrode sheet to conductors or bus bars made of copper metal or metal alloy or aluminum metal or metal alloy during battery assembly. Two types of tab materials are commonly used in battery construction: aluminum and copper. In some cases, the copper tabs and/or copper conductor may be coated with a thin layer of nickel to enhance corrosion resistance. In some cases, the aluminum tabs and/or aluminum conductor may have a thin anodization layer.
Joining the thin tab materials to the much thicker conductor has been difficult for a number of reasons. First, the stack-ups require the joining of several separate pieces of metal in one operation, e.g., multiple separate tabs to one conductor. Second, the stack-ups can include a dissimilar metal combination that is known to be difficult due to the formation of brittle intermetallics, e.g., copper and aluminum. Third, the thickness ratio between the conductor and battery cell tabs can be high, for example at least about 4:1 or more.
Ultrasonic metal welding has been used for this application with some success. It enables the joining of dissimilar metals and is capable of joining materials with significant differences in sheet thickness. However, due to the high force necessary for ultrasonic metal welding, a backstop is used to allow for proper joining of the members. This requires access to both sides of the battery tabs and the bus bars, which severely limits the ability to design more compact battery assemblies. Another shortcoming of a welded joint is that the joint cannot be easily taken apart nondestructively for replacement or service.
Mechanical fasteners have also been used. Mechanical fasteners, such as screws or clamps, provide a reversible joint. They rely on very low contact resistance to achieve good electrical conductivity. However, contact resistance can degrade over time through buildup of surface contaminants (e.g., oxides), or degradation of the fastener. Furthermore, screws or clamps incur significant mass, cost, and assembly time.
Soldered joints can also be used. However, the use of solders with fluxing agents, particularly for aluminum, can result in the formation of corrosive flux residue that will degrade the surrounding materials or joint over time if not removed by cleaning operations. These operations add cost and, in some cases, may not be possible depending on the assembly sequence.
Laser welding is another process that has seen use more recently. This process allows for strong, consistent welds within small areas. The major disadvantage of laser welding is the use of fusion which does not work well for dissimilar metal welding without bi-metal bus bars. Also, the high monetary cost, which can be prohibitive if only small quantities are needed.
Due to its limited power capacity, wire and ribbon bonding have traditionally been used primarily in low electrical load applications, such as semiconductor packaging. While being used by certain battery manufacturers, this method is not commonly used for joining battery tabs to bus bars in automotive battery manufacture. While ribbon bonding does not require the high forces of ultrasonic metal welding and is less expensive than laser welding, its own limitations are present. The process uses a bonding ribbon or wire to join the structural elements, which is less effective in the present application. It would also be very difficult to bond the bus bar and cell tabs across an edge, and the extra layer of material would drastically reduce the effectiveness of the weld.
There remains a need for a process for joining battery cell tabs to conductors or bus bars.